1. Technical Field
The present disclosure relates to a communication device, and more particularly to a terminal and a base station, and to a signal transmitting method and a signal receiving method in a terminal and a base station, respectively.
2. Description of the Related Art
In 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), orthogonal frequency division multiple access (OFDMA) is used as a downlink communication method.
In a wireless communication system using 3GPP LTE, hybrid automatic repeat request (HARQ) is applied to downlink data transmitted from a base station (may be referred to as “eNB”) to a terminal (may be referred to as “user equipment (UE)”). That is, a terminal feeds back error detection results concerning downlink data to the base station as a response signal. The terminal performs a cyclic redundancy check (CRC) on downlink data. If there is no error in CRC calculation results, the terminal returns acknowledgement (ACK) to the base station as a response signal. If there is any error in CRC calculation results, the terminal returns negative acknowledgement (NACK) to the base station as a response signal. For feeding back error detection results as a response signal (that is, ACK/NACK signal), an uplink control channel such as a physical uplink control channel (PUCCH) is used.
In 3GPP LTE, as shown in FIG. 1, multiple ACK/NACK signals transmitted from plural terminals are spread on the time domain by using zero auto-correction (ZAC) sequences having ZAC characteristics (multiplied by ZAC sequences), and are subjected to code-multiplexing in PUCCH (see, for example, 3GPP TS 36.211 V11.5.0, “Physical channels and modulation (Release 11)”, December 2013, 3GPP TS 36.212 V11.4.0, “Multiplexing and channel coding (Release 11)”, December 2013, and 3GPP TS 36.213 V11.5.0, “Physical layer procedures (Release 11)”, December 2013). In FIG. 1, (W(0), W(1), W(2), W(3)) represent Walsh sequences of a sequence length 4, while (F(0), F(1), F(2)) represent discrete Fourier transform (DFT) sequences of a sequence length 3.
As shown in FIG. 1, in a terminal, an ACK/NACK signal is first subjected to primary spreading in which the ACK/NACK signal is spread into frequency components each corresponding to one single-carrier frequency division multiple access (SC-FDMA) symbol on the frequency domain by using a ZAC sequence (sequence length 12). That is, ACK/NACK signal components represented by a complex number are multiplied by the ZAC sequence (sequence length 12). Then, the ACK/NACK signal subjected to primary spreading and the ZAC sequence as a reference signal are subjected to secondary spreading by using a Walsh sequence (sequence length 4: W(0) through W(3)) and a DFT sequence (sequence length 3: F(0) through F(2)). That is, each component forming the signal of a sequence length 12 (ACK/NACK signal subjected to primary spreading or ZAC sequence as a reference signal) is multiplied by a corresponding one of components of an orthogonal sequence (Walsh sequence or DFT sequence). Then, the signal subjected to secondary spreading is converted into a signal of a sequence length 12 on the time domain by using inverse discrete Fourier transform (IDFT) or inverse fast Fourier transform (IFFT). Then, a cyclic prefix (CP) is added to each component of the resulting signal. As a result, one slot signal constituted by seven SC-FDMA symbols is formed.
PUCCH resources are allocated to each terminal in units of subframes. One subframe is constituted by two slots.
ACK/NACK signals transmitted from different terminals are spread (multiplied) by ZAC sequences defined by different cyclic shift indexes or orthogonal sequences corresponding to different orthogonal cover indexes (OC indexes). The orthogonal sequences are constituted by a combination of Walsh sequences and DFT sequences. The orthogonal sequence may also be referred to as a “block-wise spreading code”. Accordingly, by performing despread processing and correlation processing, the base station is able to separate the multiple ACK/NACK signals subjected to code-multiplexing from each other (see, for example, Seigo Nakao, Tomofumi Takata, Daichi Imamura, and Katsuhiko Hiramatsu, “Performance enhancement of E-UTRA uplink control channel in fast fading environments”, Proceeding of 2009 IEEE 69th Vehicular Technology Conference (VTC2009-Spring), April 2009). FIG. 2 illustrates PUCCH resources defined by OC indexes 0 through 2 of orthogonal sequences and cyclic shift indexes 0 through 11 of ZAC sequences. If Walsh sequences of a sequence length 4 and DFT sequences of a sequence length 3 are used, a maximum of 3×12=36 PUCCH resources can be defined in the same time-frequency resources. However, it is not always possible that all the 36 PUCCH resources be used. In FIG. 2, for example, for suppressing the transmission performance degradation caused by a timing offset at a terminal, delay spread due to multipath propagation, and inter-code interference due to the movement of a terminal, 18 PUCCH resources (#0 through #17) are used.
As a system for supporting future information society, machine-to-machine (M2M) communication is promising. In M2M communication, services can be provided by autonomous communication between devices without the need of user's judgement. One of the specific applications of the M2M communication system is a smart grid. The smart grid is an infrastructure system for efficiently supplying energy resources such as electricity and gas. For example, the smart grid performs M2M communication between smart meters installed in households and buildings and a central server so as to autonomously and effectively adjust the balance of supply and demand of resources. Other specific applications of the M2M communication system are a monitoring system for commodity control or telemedicine and a remote control system for the stock and billing of vending machines, for example.
In the M2M communication system, the use of a cellular system having a wide communication area is attracting people's attention. In 3GPP, in the LTE and LTE-Advanced standards, M2M based on a cellular network is being developed as the name of machine type communication (MTC). If MTC communication devices such as smart meters are installed in certain locations, such as the basement of a building, they may not be used in an existing communication area. For dealing with such a situation, coverage enhancement for further increasing the communication area is being considered (see, for example, 3GPP TR 36.888 V12.0.0, “Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE”, June 2013).
In MTC coverage enhancement, for further increasing the communication area, repetition transmission in which the same signal is transmitted multiple times, more specifically, the execution of repetition transmission in PUCCH, is being considered. In a base station, which is a receiving side of PUCCH, by combining repeatedly transmitted signals, received signal power can be enhanced, thereby making it possible to increase a communication area.